Anaerobic micoorganisms generate reducing equivalents such as quinones, NADH etc. during oxidative metabolic processes, for example glycolysis. These reducing equivalents can transfer electrons to conductive materials, such as an anode, and thereby change the redox state of the conductive material. This phenomenon allows the accumulation of negative electric charge from microbial processes at a collector. Several processes based on oxygen reduction are known which lead to the accumulation of positive electric charge on another conductive material, such as a cathode. By closing a circuit between the conductive materials, an electric current can be generated. This basic idea is realised, more or less successfully, in various types of devices called microbial fuel cells.
Cathode assemblies for microbial fuel cells usually differ from cathode assemblies for chemical fuel cells. A major constraint which affects the design of microbial fuel cell cathode assemblies follows from the requirement for the cathodes to function alongside biological processes located in an anodic zone, while minimising or obviating chemical effects upon bacterial cells. Due to this restriction, the reaction which takes place at the cathode of microbial fuel cells is typically based on the reduction of oxygen from air. This is commercially attractive because oxygen is freely available in air.
Nevertheless, in some cathode assemblies, power consumption is required to supply oxygen to the catalyst. For example, in so-called immersed cathode systems where the catalyst is wholly immersed in catholyte meaning that oxygen must diffuse through the catholyte, energy is typically consumed to pump oxygen to the immersed cathode or to pump oxygen saturated catholyte across the surface of the cathode. The power consumption of these pumping mechanisms can exceed the power output of the bacteria in the anodic zone.
An alternative known type of cathode is a so-called “air facing cathode” (also referred to as a “dry cathode”) which is in the form of an electrolyte-permeable sheet having a first surface in contact with the aqueous electrolyte of the cell and an opposed second surface which is exposed to atmospheric air. As oxygen does not need to diffuse through the catholyte, the mass transport of oxygen is less of a limiting factor than for an immersed cathode. However, water accumulates in the electrolyte-permeable sheet both from the aqueous catholyte and the formation of water as a result of the reduction of oxygen. Some of the water evaporates, but no water forms on the surface of the electrolyte-permeable sheet. Solid salts typically precipitate within the electrolyte-permeable sheets, due to ion exchange between the anodic and cathodic zones, blocking the cathode during long term use in open circuit mode.
Several approaches have been proposed to improve these known cathode assemblies.
US 2005/208343 (Korea Institute of Science and Technology) discloses a membrane-less microbial fuel cell comprising a simple air cathode. The air cathode is located near the top of a cylindrical anaerobic reactor (containing an anode) which is adapted such that the distance between the anode and the cathode may be varied. Waste water, which contains organic species, is fed through an inlet at the bottom of the reactor from where it passes upwards. Cleaned effluent exits the reactor from the top of the uppermost chamber.
This arrangement has several advantages. The distance between the anode and the cathode affects electric current generation, therefore this type of cathodic arrangement provides a variable performance microbial fuel cell. This design of microbial fuel cell can readily be scaled up. Also, no special cathodic electrolyte is required as treated waste water exits from the anodic zone and passes upwards into the cathodic zone, where it is used as cathodic electrolyte. Nevertheless, the efficiency of the air cathode is very low because the absence of a membrane between the anodic and cathodic zones which leads to a loss of oxygen and positive electric charges, due to the reduction in the cathodic zone by means of reducing equivalents, which were produced by microbes in the anodic zone.
WO 2006/072112 (Washington University) discloses a mediator-less fuel cell in which the cathodes take the form of U-shape conductive tubes. The outside surface of a cathodic tube includes an ion-exchange membrane. Cathodic potential is generated by feeding potassium ferricyanide through the cathodic tube. The cathodic tubes functions as collectors of positive charge and the interior of the tube is filled with granules of activated carbon, which are used as anodic electrodes. This arrangement of cathodic tubes improves cathode performance due to the increased total surface area of the composite electrode. However, the net energy balance of microbial fuel cells which use ferricyanides is negative. Moreover, ferricyanides may work effectively in alkaline electrolytes, but during long term experiments, in current generation mode, the pH of the cathodic zone slowly drops down due to the proton flux from the anodic zone.
Zhao et al., (In Electrochemistry Communications, Vol. 7 (2005), p. 1405) tested immersed graphite cathodes where metallo-organic catalysts such as iron and cobalt phthalocyanines and cobalt tetramethoxyphenylporphyrine (CoTMPP) were used as platinum substitutes. It was found that phthalocyanines can give almost the same current densities as platinum but have the advantage that they are cheaper. However, like all immersed cathodes, they have a problem with mass transport of oxygen to the electrode and they accumulate hydrogen peroxide during long term work in current generation mode. The attempt to increase mass transport by intensifying the mixing conditions increase the energy consumption associated with aeration.
Park and Zeikus (Biotechnology and Bioengineering, Vol. 81 (2003), p. 348) have proposed an improved air cathode based on the reaction: O2+4H++4e−2H2O, in which iron ions serve as the catalyst. The electrode comprises a kaolin matrix containing dispersed iron. In such electrodes, the process of water evaporation from the electrode surface would be expected to compete with the process of water accumulation in the porous electrode system. This phenomenon could take place when the current is low, and this imbalance reduces the efficiency of the cathode.
S. Cheng et al., (Electrochemistry Communications, Vol. 8 (2006), p. 489) disclosed an improved on air cathode based on the reaction: O2+4H++4e−2H2O, which included a carbon cloth electrode coated on the air-facing side with a mixture of polytetrafluoroethylene (PTFE) and carbon powder to prevent water losses. A platinum catalyst was applied to the water-facing side using Nafion as a binder. (Nafion is a trade mark) However, due to the use of a proton permeable Nafion membrane, it was not possible to obtain good current densities, for example, more 1 mA/cm2 at cathodic potential equal to +200 mv relative to a standard hydrogen electrode because the pH of the biological anodic zone was in the range of 5.0-5.5. Moreover, at high current densities the proton concentration limits the rate of the cathodic reaction. Furthermore, the use of platinum as cathodic catalyst is commercially unattractive, due to its cost.
The invention aims to provide a microbial fuel cell cathode assembly with improvements relating to one or more of current density generation, operating lifetime, oxygen transport to the cathodic catalyst, electrical capacity per unit mass and equilibrium potential.